1. Field of the Invention
This invention relates to hoses suitable for use in transporting or otherwise handling refrigerant and oils fluids.
2. Prior Art
Known hoses of this type are comprised of an inner tube and an outer cover, the inner tube being of a two-layered structure having a thermoplastic, resinous wall and a rubbery wall so as to improve impermeation to gaseous and oily fluids. An inner wall of the hose is usually formed by a thermoplastic material having regard to chemical and heat resistance.
In this prior construction, however, the inner tube is difficult to tightly interconnect with a metal fitting under clamping force because the inner resinous wall of the tube is great in modulus, generally about 10 to 500 times the outer cover and the outer wall. Excessive clamping would deform or eventually impair the outer cover and even the outer wall, leading to hazardous leakage of a refrigerant or oil at from the hose portion so affected. Insufficient clamping would make the hose ready to slip out of the metal fitting and hence susceptible to cutting. To cope with the foregoing problems, it has been proposed to apply an adhesive or a rubber coating onto the fitting. This is literally tedious but with little success.
Certain resins are known in common use as materials for the inner tube, and they include nylon-6, a copolymer of nylon-6 and nylon-66 (nylon-6-66), nylon-11 and nylon-12. Nylon-6 and nylon-6-66 have been found satisfactory with respect to gas impermeation but susceptible to stress cracking and rigidity, while nylon-11 and nylon-12 are not sufficiently impermeable to gas.
It has also been found that if pin holes or minute scars occur on an inner resinous wall of the inner tube during tube processing as by extrusion molding or during fabrication of the hose, then they tend to grow into cracks due to repeated compression of the hose in service, or into stress cracks particularly on contact with metal chlorides such as zinc chloride. This type of cracking would in most instances extend to and break an outer rubbery wall as is usually called crack propagation and thus leak a fluid in transit, resulting in bursted hose and hence reduced service life.